The kinetic and potential energies are both different forms of the same basic quantity, i.e. mechanical energy. Total mechanical energy of a body is the sum of the kinetic energy and potential energy may change into kinetic energy and vice versa, but the total energy remains constant. Mathematically,

Total energy = P.E. + K.E. = constant

This is a special case of the law of conservation of energy which states that:

 For your information Source of energy Original source solar Sun Bio mass Sun Fossil fuels Sun Wind Sun Hydro electric Sun Tides Moon Geothermal Earth

Energy cannot be destroyed. It can be transformed from one kind into another, but the total amount of energy remains constant.

This is one of the basic laws of physics. We daily observe many energy transformations from one form to another. Some forms, such as electrical and chemical energy, are more easily transferred than others, such as heat. Ultimately all energy transfers result in heating of the environment and energy is wasted. For example, the P.E. of the falling object changes to K.E., but on striking the ground, the K.E. changes into heat and sound. If it seems in an energy transfer that some energy has disappeared, the lost energy is often converted into heat. This appears to be the fate of all available energies and is one reason why new sources of useful energy have to be developed.

Example 4.3:

A brick for mass 2.0 kg is dropped from a rest position 5.0 m above the ground. What is its velocity at a height of 3.0 m above the ground?

Solution: Using Eq. 4.19

 Energy sources Renewable Nonrenewable HydroelectricWind Tides Geothermal * Biomass Sunlight Ethanol/methanol** Coal Natural gas Oil Uranium Oil shale Tar sands *individual fields may run off** Renewable when made from bio mass.